A known apparatus for inflating an inflatable vehicle occupant restraint, such as an air bag, includes a pressure vessel for storing a quantity of inflation fluid under pressure. The pressure vessel includes a burst disk which is ruptured in response to vehicle deceleration of a magnitude indicating the occurrence of a collision. When the burst disk is ruptured, it defines an outlet opening through which the inflation fluid flows from the pressure vessel toward the vehicle occupant restraint to inflate the vehicle occupant restraint. The area of the opening defined by the ruptured burst disk is a factor in determining the rate at which the inflation fluid flows into the vehicle occupant restraint.